Atomic layer deposition method of depositing an oxide on a substrate

ABSTRACT

The invention includes atomic layer deposition methods of depositing an oxide on a substrate. In one implementation, a substrate is positioned within a deposition chamber. A first species is chemisorbed onto the substrate to form a first species monolayer within the deposition chamber from a gaseous precursor. The chemisorbed first species is contacted with remote plasma oxygen derived at least in part from at least one of O 2  and O 3  and with remote plasma nitrogen effective to react with the first species to form a monolayer comprising an oxide of a component of the first species monolayer. The chemisorbing and the contacting with remote plasma oxygen and with remote plasma nitrogen are successively repeated effective to form porous oxide on the substrate. Other aspects and implementations are contemplated.

TECHNICAL FIELD

This invention relates to atomic layer deposition methods of depositingan oxide on a substrate.

BACKGROUND OF THE INVENTION

Integrated circuits are typically formed on a semiconductor substratesuch as a silicon wafer or other semiconductive material. In general,layers of various materials, which are one of semiconductive, conductingor insulating, are used to form the integrated circuits. By way ofexample, the various materials are doped, ion implanted, deposited,etched, grown, etc., using various processes. A continuing goal insemiconductor processing is to reduce the size of individual electroniccomponents, thereby enabling smaller and denser integrated circuitry.

As semiconductor devices continue to shrink geometrically, such has hada tendency to result in greater shrinkage in the horizontal dimensionthan in the vertical dimension. In some instances, the verticaldimension increases. Regardless, the result is increased aspect ratios(height to width) of the devices, making it increasingly important todevelop processes that enable materials to conformally deposit over thesurfaces of high aspect ratio features. One such processing is atomiclayer deposition, which involves the deposition of successive monolayersover a substrate within a deposition chamber typically maintained atsubatmospheric pressure. With typical atomic layer deposition,successive mono-atomic layers are adsorbed to a substrate and/or reactedwith the outer layer on the substrate, typically by the successivefeeding of different deposition precursors to the substrate surface.

One commonly used class of materials in the fabrication of integratedcircuitry is oxides. Some oxides are electrically conductive, whileother oxides are electrically insulative.

While the invention was motivated in addressing the above issues, it isin no way so limited. The invention is only limited by the accompanyingclaims as literally worded, without interpretative or other limitingreference to the specification, and in accordance with the doctrine ofequivalents.

SUMMARY

The invention includes atomic layer deposition methods of depositing anoxide on a substrate. In one implementation, a substrate is positionedwithin a deposition chamber. A first species is chemisorbed onto thesubstrate to form a first species monolayer within the depositionchamber from a gaseous precursor. The chemisorbed first species iscontacted with remote plasma oxygen derived at least in part from atleast one of O₂ and O₃ and with remote plasma nitrogen effective toreact with the first species to form a monolayer comprising an oxide ofa component of the first species monolayer. The chemisorbing and thecontacting with remote plasma oxygen and with remote plasma nitrogen aresuccessively repeated effective to form porous oxide on the substrate.

Other aspects and implementations are contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic depiction of a substrate in process inaccordance with an aspect of the invention.

FIG. 2 is a view of the FIG. 1 substrate at a processing step subsequentto that depicted by FIG. 1.

FIG. 3 is a view of the FIG. 2 substrate at a processing step subsequentto that depicted by FIG. 2.

FIG. 4 is a view of the FIG. 3 substrate at a processing step subsequentto that depicted by FIG. 3.

FIG. 5 is a view of the FIG. 4 substrate at a processing step subsequentto that depicted by FIG. 4.

FIG. 6 is an alternate view of the FIG. 3 substrate at a processing stepsubsequent to that depicted by FIG. 3.

FIG. 7 is a view of the FIG. 6 substrate at a processing step subsequentto that depicted by FIG. 6.

FIG. 8 is a view of the FIG. 7 substrate at a processing step subsequentto that depicted by FIG. 7.

FIG. 9 is a diagrammatic depiction of gas flow as a function of time.

FIG. 10 is an alternate diagrammatic depiction of gas flow as a functionof time.

FIG. 11 is an alternate diagrammatic depiction of gas flow as a functionof time.

FIG. 12 is a diagrammatic depiction of a system usable in accordancewith an aspect of the invention.

FIG. 13 is an alternate diagrammatic depiction of a system usable inaccordance with an aspect of the invention.

FIG. 14 is an alternate diagrammatic depiction of a system usable inaccordance with an aspect of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

The invention comprises atomic layer deposition methods of depositingoxides on substrates. Atomic layer depositing (ALD) typically involvesformation of successive atomic layers on a substrate. Described insummary, ALD includes exposing an initial substrate to a first chemicalspecies to accomplish chemisorbtion of the species onto the substrate.Theoretically, the chemisorbtion forms a monolayer that is uniformly oneatom or molecule thick on the entire exposed initial substrate. In otherwords, a saturated monolayer is preferably formed. Practically,chemisorbtion might not occur on all portions or completely over thedesired substrate surfaces. Nevertheless, such an imperfect monolayer isstill considered a monolayer in the context of this document. In manyapplications, merely a substantially saturated monolayer may besuitable. A substantially saturated monolayer is one that will stillyield a deposited layer exhibiting the quality and/or properties desiredfor such layer.

The first species is purged from over the substrate and a secondchemical species is provided to chemisorb onto the first monolayer ofthe first species. The second species is then purged and the steps arerepeated with exposure of the second species monolayer to the firstspecies. In some cases, the two monolayers may be of the same species.Also, a third species or more may be successively chemisorbed and purgedjust as described for the first and second species. Further, one or moreof the first, second and third species can be mixed with inert gas tospeed up pressure saturation within a reaction chamber.

Purging may involve a variety of techniques including, but not limitedto, contacting the substrate and/or monolayer with a carrier gas and/orlowering pressure to below the deposition pressure to reduce theconcentration of a species contacting the substrate and/or chemisorbedspecies. Examples of carrier gases include nitrogen, Ar, He, Ne, Kr, Xe,etc. Purging may instead include contacting the substrate and/ormonolayer with any substance that allows chemisorption byproducts todesorb and reduces the concentration of a species preparatory tointroducing another species. A suitable amount of purging can bedetermined experimentally as known to those skilled in the art. Purgingtime may be successively reduced to a purge time that yields an increasein film growth rate. The increase in film growth rate might be anindication of a change to a non-ALD process regime and may be used toestablish a purge time limit.

ALD is often described as a self-limiting process in that a finitenumber of sites exist on a substrate to which the first species may formchemical bonds. The second species might only bond to the first speciesand thus may also be self-limiting. Once all of the finite number ofsites on a substrate are bonded with a first species, the first specieswill often not bond to other of the first species already bonded withthe substrate. However, process conditions can be varied in ALD topromote such bonding and render ALD not self-limiting. Accordingly, ALDmay also encompass a species forming other than one monolayer at a timeby stacking of a species, forming a layer more than one atom or moleculethick. Further, local chemical reactions can occur during ALD (forinstance, an incoming reactant molecule can displace a molecule from anexisting surface rather than forming a monolayer over the surface). Tothe extent that such chemical reactions occur, they are generallyconfined within the uppermost monolayer of a surface.

Traditional ALD can occur within frequently-used ranges of temperatureand pressure and according to established purging criteria to achievethe desired formation of an overall ALD layer one monolayer at a time.Even so, ALD conditions can vary greatly depending on the particularprecursors, layer composition, deposition equipment, and other factorsaccording to criteria known by those skilled in the art. Maintaining thetraditional conditions of temperature, pressure, and purging minimizesunwanted reactions that may impact monolayer formation and quality ofthe resulting overall ALD layer. Accordingly, operating outside thetraditional temperature and pressure ranges may risk formation ofdefective monolayers.

An exemplary preferred embodiment is initially described with referenceto FIGS. 1-5. Referring to FIG. 1, there diagrammatically depicted is asubstrate fragment 10. In one exemplary embodiment, substrate fragment10 is a semiconductor substrate, for example comprising a substrate 12comprising some material which preferably includes some semiconductivematerial and may, of course, include multiple materials and layers. Inthe context of this document, the term “semiconductor substrate” or“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove. Substrate 12 is positioned within a suitable deposition chamber.A first species AB has been chemisorbed to form a first speciesmonolayer 14 onto substrate 12 within the deposition chamber from asuitable gaseous precursor.

Referring to FIGS. 2 and 3, chemisorbed first species AB has beencontacted with remote plasma oxygen derived at least in part from O₂ andO₃ and with remote plasma nitrogen (FIG. 2) effective to react withfirst species AB to form a monolayer 16 (FIG. 3) comprising an oxide AOof a component (i.e., A) of the first species monolayer. Thechemisorbing and the contacting with remote plasma oxygen and withremote plasma nitrogen are successively repeated effective to formporous oxide on the substrate. The same or different gaseous precursorsfor the stated chemisorbings and contactings can be utilized. It hasbeen discovered that the provision of remote plasma nitrogen (even andpreferably in small amounts) has the effect of forming the oxide to beporous, whereas otherwise identical processing in the absence of remoteplasma nitrogen does not result in any appreciable porosity in the oxidelayer formed.

FIG. 4 depicts exemplary repeating of chemisorbing of first species ABto form a first species monolayer 14′ onto oxide AO. FIG. 5 depictssuccessive contacting with remote plasma oxygen and remote plasmanitrogen effective to form a monolayer 16′ comprising oxide AO of acomponent of first species monolayer 14′.

The porous oxide which is formed might be an electrically insulativeoxide or an electrically conductive oxide. By way of example only,exemplary preferred insulative oxides include SiO₂ and Al₂O₃. Further byway of example only, exemplary preferred electrically conductive oxidesinclude tin oxide and indium oxide. By way of example only, for theformation of porous Al₂O₃, an exemplary gaseous precursor forchemisorbing the first species is trimethyl aluminum, with a likelyfirst species being aluminum chemisorbed to the substrate, with methylgroups pending from the chemisorbed aluminum. Accordingly, thecontacting with remote plasma oxygen and with remote plasma nitrogenforms an oxide with an aluminum component of the first speciesmonolayer.

By way of example only, where the porous oxide comprises SiO₂, anexemplary gaseous precursor comprises at least one oftetraethylorthosilicate (TEOS) and a silane. Accordingly, thechemisorbed first species will comprise silicon, which will combine withthe remote plasma oxygen to form an oxide. Further by way of exampleonly, where the porous oxide comprises tin oxide, an exemplary gaseousprecursor is trimethyl tin. Accordingly, the chemisorbed first specieswill comprise tin, which combines with the remote plasma oxygen to formtin oxide. Further by way of example only, where the porous oxidecomprises indium oxide, an exemplary gaseous precursor is trimethylindium. Accordingly, the chemisorbed first species will comprise indium,which combines with the remote plasma oxygen to form indium oxide.

Further by way of example only, the porous oxide might comprise multiplecations. By way of example only, one such porous oxide comprisesIn_(x)Sn_(y)O, which is an electrically conductive oxide. By way ofexample only, exemplary indium and tin precursors are trimethyl tin andtrimethyl indium. Such might be fed to the deposition chambersimultaneously in desired ratios towards achieving the desired quantityof tin and indium in the oxide being formed. Further by way of exampleonly, such simultaneous feeding might be as a mixture or separately tothe chamber. Further by way of example only, the tin containingprecursor and the indium containing precursor might be fed to thedeposition chamber at different times. Such different times mightoverlap with one another, or be spaced from one another such that thereis no overlap.

By way of example only, FIG. 6 diagrammatically depicts an alternateembodiment substrate fragment 10 a. Like numerals from the firstdescribed embodiment are utilized where appropriate, with differencesbeing designated with different letters or with the suffix “a”.Substrate fragment 10 a is shown at a processing step subsequent to thatdepicted by FIG. 3 with respect to substrate fragment 10. A firstspecies DB has been chemisorbed to form a first species monolayer 14 aonto the substrate, here onto species AO, within the deposition chamberfrom a suitable gaseous precursor. For example, and by way of exampleonly, oxide AO might comprise one or the other of tin oxide and indiumoxide, with first species DB comprising the other of a tin or indiumcontaining species, and with “D” designating one of the tin or indium,and the “A” designating the other of tin or indium.

Referring to FIGS. 7 and 8, chemisorbed first species DB is contactedwith remote plasma oxygen and with remote plasma nitrogen (FIG. 7)effective to react with first species DB to form a monolayer 16 a (FIG.8) comprising an oxide DO of a component (i.e., component D) of firstspecies monolayer 14 a. Of course, the illustrated monolayers 16 and 16a might respectively include combinations of the A and D exemplifiedoxides, as well as the same oxide molecules bonding to one another.

Exemplary preferred remote plasma nitrogen is derived from any one orcombination of N₂, N₂O and NO_(x). Further, the remote plasma oxygenmight comprise one or a combination of O₂ and O₃.

The remote plasma oxygen and the remote plasma nitrogen are mostpreferably fed to the deposition chamber simultaneously, meaning overthe same identical time interval (for example as depicted in FIG. 9).Alternately, the remote plasma oxygen and the remote plasma nitrogen arefed separately to the deposition chamber at different times. Thedifferent times might overlap one another (for example as depicted inFIG. 10), or the different times might be spaced from one another suchthat they do not overlap (for example as depicted in FIG. 11). Furtherif at different times, either could precede the other.

In one preferred implementation, the remote plasma oxygen and the remoteplasma nitrogen are fed as a mixture to the deposition chamber. FIG. 12depicts a most preferred and reduction-to-practice embodiment 30. Anitrogen stream and a stream comprising at least one of O₂ and O₃ aremixed and fed to an ozone generator OG. Ozone generator OG can be “off”or “on”, and if “on” will convert a portion of the O₂ to O₃. Regardless,the mixed stream is depicted as then feeding to a remote plasmagenerator RPG wherein a remote plasma mixture is generated. Accordinglyin this depicted embodiment, remote plasma oxygen and remote plasmanitrogen are generated in the same remote plasma generating chamber.Such is then depicted as being fed as a mixture to a deposition chamberDC within which the substrate being deposited upon is received. Agaseous feed line X is depicted downstream of remote plasma generatorRPG. More streams might be provided, as well as before or after remoteplasma generator RPG. Such could constitute one or more other gaseousprecursor streams and/or inert purge gas or carrier gas streams.

By way of example only, an alternate lesser preferred embodiment system40 is depicted in FIG. 13. Here, remote plasma oxygen derived at leastfrom at least one of O₂ and O₃ and remote plasma nitrogen are generatedin different remote plasma generating chambers RPG and then mixed priorto feeding to deposition chamber DC. Further by way of example only,FIG. 14 depicts another alternate embodiment system 50. Here, the remoteplasma oxygen and the remote plasma nitrogen are fed separately to thedeposition chamber, for example either simultaneously or at differenttimes.

Regardless of whether mixing or fed separately, when the remote plasmaoxygen and the remote plasma nitrogen are fed to the deposition chamberat the same time, a preferred quantity or concentration of the remoteplasma nitrogen is from 0.01% to 90% by volume of all remote plasmaoxygen and remote plasma nitrogen fed to the deposition chamber. Morepreferably, the remote plasma nitrogen is from 0.1% to 10%, and morepreferably from 0.1% to 3% by volume of all remote plasma oxygen andremote plasma nitrogen fed to the deposition chamber. Very lowquantities of remote plasma nitrogen have been found effective toproduce porous oxide in accordance with the invention, with anotherpreferred concentration range for the remote plasma nitrogen being from0.01% to 1% by volume of all remote plasma oxygen and remote plasmanitrogen fed to the deposition chamber.

An exemplary preferred temperature range for the stated chemisorbingsand contactings is from 100° C. to 500° C., and more preferably from200° C. to 350° C. Exemplary preferred pressure within the chamberduring the stated chemisorbings and contactings is from 200 mTorr to 10Torr, with from 500 mTorr to 2.5 Torr being more preferred. An exemplarypreferred remote plasma generating power is from 1000 watts to 6000watts, with 4000 watts being a specific example. An exemplary preferredfrequency for the remote plasma generation is 13.5 mHz, with such powersand frequencies being for an exemplary total two liters feed of gases tothe remote plasma generator. Of course, some of the remote plasmamaterial generated might be diverted to not flow to the chamber.Further, the chamber might, of course, be a cold wall reactor or a hotwall reactor.

The invention was reduced-to-practice utilizing the embodiment system ofFIG. 12 with O₂, N₂, and trimethyl aluminum in the formation of Al₂O₃.Average temperature and pressure during the chemisorbings andcontactings was 300° C. and 1 Torr, respectively. Remote plasma powerwas 4000 watts at a frequency of 13.5 mHz for a flow rate of two litersper minute of a mixture of N₂ and O₂ to the ozone generator and remoteplasma generator, with 10 sccm thereof being nitrogen, providing a ratioof remote plasma nitrogen at 0.5% by volume of all remote plasma oxygenand remote plasma nitrogen fed to the deposition chamber. The trimethylaluminum pulsings were at a flow rate of 30 sccm for from 0.8 second to2 seconds, followed by purging with an inert gas at from 500 sccm to1000 sccm for from 5 seconds to 10 seconds, followed by feeding of themixture of remote plasma nitrogen and remote plasma oxygen for from 6seconds to 10 seconds, followed by purging with an inert gas at from 500sccm to 1000 sccm for from 5 seconds to 10 seconds. Such processing wassuccessively repeated, resulting in approximately 1.2 Angstroms of oxideformation per complete cycle. The oxide that formed had approximately50% porosity with substantially close-celled pores. The degree ofporosity is expected to be modifiable by manipulating various of theabove-stated exemplary parameters. Without being limited by any theoryof invention, it is believed that the remote plasma nitrogen might becontributing to the formation of some nitride material which then reactswith oxygen to substantially remove nitrogen from being formed in theoxide layer and generating pores in the oxide in the process.

The invention was reduced-to-practice and considered in the fabricationof integrated circuitry in semiconductor processing. However, porousoxides have other uses beyond integrated circuitry fabrication, with theinvention not being so limited. By way of example only, exemplary usesfor the technology would be to insulate sidewalls of deep structures, orsteep sidewalls and steep steps, or in forming very low k insulatorlayers as well as conductive layers, in integrated circuitryfabrication. Further, porous oxides formed in accordance with theinvention may have a myriad of other uses, whether existing or yet-to-bedeveloped, for example as anti-reflective coatings in semiconductorprocessing or on display screens; as coatings in photolithography; asmechanical, chemical or chemical/mechanical fluid filters; as catalystsupport beds, etc.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents. For example and by way of example only, the invention doesnot preclude and contemplates combination of the claimed atomic layerdepositing with other deposition methods before or after the claimedatomic layer depositing in forming porous oxide on the substrate.

1. An atomic layer deposition method of depositing an oxide on asubstrate comprising: providing a substrate within a deposition chamber;chemisorbing a first species to form a first species monolayer onto thesubstrate within the deposition chamber from a gaseous precursor;contacting the chemisorbed first species with remote plasma oxygenderived at least in part from at least one of O₂ and O₃ and with remoteplasma nitrogen effective to react with the first species to form amonolayer comprising an oxide of a component of the first speciesmonolayer; and successively repeating the chemisorbing and thecontacting with remote plasma oxygen and with remote plasma nitrogeneffective to form porous electrically conductive oxide comprisingIn_(x)Sn_(y)O, on the substrate, the gaseous precursor comprising anindium-containing precursor and a tin-containing precursor which are fedto the deposition chamber simultaneously.
 2. An atomic layer depositionmethod of depositing an oxide on a substrate comprising: providing asubstrate within a deposition chamber; chemisorbing a first species toform a first species monolayer onto the substrate within the depositionchamber from a gaseous precursor; contacting the chemisorbed firstspecies with remote plasma oxygen derived at least in part from at leastone of O₂ and O₃ and with remote plasma nitrogen effective to react withthe first species to form a monolayer comprising an oxide of a componentof the first species monolayer; and successively repeating thechemisorbing and the contacting with remote plasma oxygen and withremote plasma nitrogen effective to form porous electrically conductiveoxide comprising In_(x)Sn_(y)O, on the substrate, the gaseous precursorcomprising an indium-containing precursor and a tin-containing precursorwhich are fed to the deposition chamber at different times.
 3. Themethod of claim 2 wherein the different times overlap one another. 4.The method of claim 2 wherein the different times are spaced from oneanother.
 5. An atomic layer deposition method of depositing an oxide ona substrate comprising: providing a substrate within a depositionchamber; chemisorbing a first species to form a first species monolayeronto the substrate within the deposition chamber from a gaseousprecursor; contacting the chemisorbed first species with remote plasmaoxygen derived at least in part from at least one of O₂ and O₃ and withremote plasma nitrogen effective to react with the first species to forma monolayer comprising an oxide of a component of the first speciesmonolayer, the remote plasma oxygen and the remote plasma nitrogen beingfed separately to the deposition chamber simultaneously; andsuccessively repeating the chemisorbing and the contacting with remoteplasma oxygen and with remote plasma nitrogen effective to form porousoxide on the substrate.
 6. An atomic layer deposition method ofdepositing an oxide on a substrate comprising: providing a substratewithin a deposition chamber; chemisorbing a first species to form afirst species monolayer onto the substrate within the deposition chamberfrom a gaseous precursor; contacting the chemisorbed first species withremote plasma oxygen derived at least in part from at least one of O₂and O₃ and with remote plasma nitrogen effective to react with the firstspecies to form a monolayer comprising an oxide of a component of thefirst species monolayer, the remote plasma oxygen and the remote plasmanitrogen being generated in different remote plasma generating chambers;and successively repeating the chemisorbing and the contacting withremote plasma oxygen and with remote plasma nitrogen effective to formporous oxide on the substrate.
 7. The method of claim 6 wherein theremote plasma oxygen and the remote plasma nitrogen are fed as a mixtureto the deposition chamber.
 8. An atomic layer deposition method ofdepositing an oxide on a substrate comprising: providing a substratewithin a deposition chamber; chemisorbing a first species to form afirst species monolayer onto the substrate within the deposition chamberfrom a gaseous precursor; feeding a) at least one of O₂ and O₃, and b)nitrogen to a remote plasma generator and forming a mixture of remoteplasma oxygen and remote plasma nitrogen therefrom, the mixturecomprising the remote plasma nitrogen at from 0.1% to 10% by volume ofall remote plasma oxygen and remote plasma nitrogen generated by thegenerator feeding the remote plasma mixture to the deposition chamberand to contact the chemisorbed first species effective to react with thefirst species to form a monolayer comprising an oxide of a component ofthe first species monolayer; and successively repeating the chemisorbingand the contacting with remote plasma oxygen and with remote plasmanitrogen effective to form porous oxide on the substrate, the gaseousprecursor comprising an indium-containing precursor and a tin-containingprecursor which are fed to the deposition chamber at different times. 9.An atomic layer deposition method of depositing an oxide on a substratecomprising: providing a substrate within a deposition chamber;chemisorbing a first species to form a first species monolayer onto thesubstrate within the deposition chamber from a gaseous precursor;feeding a) at least one of O₂ and O₃, and b) nitrogen to a remote plasmagenerator and forming a mixture of remote plasma oxygen and remoteplasma nitrogen therefrom, the mixture comprising the remote plasmanitrogen at from 0.1% to 10% by volume of all remote plasma oxygen andremote plasma nitrogen generated by the generator; feeding the remoteplasma mixture to the deposition chamber and to contact the chemisorbedfirst species effective to react with the first species to form amonolayer comprising an oxide of a component of the first speciesmonolayer; and successively repeating the chemisorbing and thecontacting with remote plasma oxygen and with remote plasma nitrogeneffective to form porous oxide on the substrate, the gaseous precursorcomprising an indium-containing precursor and a tin-containing precursorwhich are fed to the deposition chamber simultaneously.
 10. The methodof claim 9 wherein the different times overlap one another.
 11. Themethod of claim 9 wherein the different times are spaced from oneanother.